The principle application area for CIG has been that of visual training simulators which present scenes to an observer or trainee to allow the observer to practice some task, such as flying an airplane. In a flight simulator, a three-dimensional model of the desired "gaming area" is prepared and stored on magnetic disk or similar bulk storage media. This model is called the visual data base. The visual simulator combines an image generator with an electro-optical display system such as a cathode ray tube (CRT), light valve projector or similar display. The image generator reads in blocks of three-dimensional data from the disk and transforms this data into two-dimensional scene descriptions. The two-dimensional data are converted to analog video that is presented to the operator via the display. The generated imagery is meant to be representative of the true scenes that the operator would see if the operator were actually performing the task being simulated. The generation of display images is said to be in "real time" which is normally taken to mean 30 frames per second, as in the U.S. television standard. CIG systems are described in detail in the book entitled Computer Image Generation edited by Bruce J. Schacter and published by Wiley-Interscience (1983).
Absolute realism is not achieved in CIG systems, but fortunately the training mission can be accomplished satisfactorily despite this apparent drawback. Recent developments have, moreover, remarkably improved the degree of realism attainable. One such development exploits the fact that the high-resolution viewing area of the eye is relatively small. This high-resolution area is the fovea of the eye, which is the only area where small details may be perceived. Surrounding the fovea is a peripheral area where the resolution of detail is low but there is a high sensitivity to movement. By taking advantage of these facts, the FOV requirement for instantaneous high resolution and high detail is greatly reduced thereby reducing the design requirements of the CIG as well as the display system. The concept in the area of interest (AOI) approach in this type of display system is to provide imagery in sufficient detail and resolution in the spatial position where the trainee is placing his attention to accomplish his training task. This point of interest, the AOI, is where the trainee is concentrating his gaze, that is where the foveal region of the eye is directed by the motion of the head and eyes. The goal of the AOI display concept is to provide this imagery while not presenting any negative cues at the AOI or in the peripheral vision field of the trainee.
An example of an AOI display system is disclosed in U.S. Pat. No. 4,348,186 issued to Harvey et al. This system provides a display of computer generated images that allocates edges or other forms of resolution by proximity to the trainee's instantaneous area of interest. This is accomplished by projecting a computer generated image onto the domed screen by apparatus which aligns with the trainee's line of sight, as determined by an eye tracker and/or a head tracker. The computer image generator is programmed with a preselected scene arrangement and is responsive to operator actuated controls to simulate a flight. The generator is also responsive to head, and most preferably, eye movements of the operator to apportion definition of the image with the scene displayed. The specific apparatus disclosed includes a helmet mounted projector. The projected image comprises a larger background image of low definition and the AOI image of high definition. These two portions of the projected image are projected by the same helmet mounted projector and preferably, the portions flow together on the screen in a smooth or nearly smooth transition. The manner in which the smooth transition between the low definition and high definition display portions is accomplished is not specifically disclosed. Harvey et al suggest that the transitions between levels of detail at the regional boundaries could be carried out by varied interpolation procedures which depend upon the display system utilized.
One major problem in AOI displays is the registration, or image contiguity, across the AOI and background transition. There are basically two approaches which have been followed in the design of AOI display systems. Either the AOI imagery is superimposed over the background imagery or a hole is "cut out" of the background image into which the AOI scene is inset. The inset approach is generally favored because the superimposition method produces a bright halo-like effect that detracts from the resulting display. However, registration errors between the AOI image and the hole cutout of the background image for the AOI image to be inset causes the AOI to have a dark border on one side and the "superimposed halo" on the opposite side. In addition, the shape of the background cutout is dynamically changing as a function of AOI position. Another major problem for any AOI concept is how to transition from the high resolution AOI scene to a lower resolution background scene. The crudest form would be an abrupt transition, but this is not satisfactory and a blending in brightness is called for. This can be done electronically by creating a pixel based staircase (in luminance) for the background and a complementary matching staircase, of several pixels per background pixel, for the AOI. This gives a discrete appearance and must be done electronically which is expensive.